Soy-based adhesive cross-linked by melamine–glyoxal and epoxy resin

To develop a soy-based adhesive with good water resistance, non-toxic melamine–glyoxal resin (MG) prepared in the laboratory was used as a cross-linker of soy-based adhesive. The FT-IR and ESI-MS results showed that there was a reaction between melamine and glyoxal. The resulted –CH–OH– groups could be the possible reactive groups for the cross-linking of soy-based adhesive. The wet shear strength of soy-based plywood indicated that the water resistance of soy adhesive cross-linked by MG improved with respect to that with no cross-linker, although it was not good enough to satisfy the relative standard. With the optimized preparation procedures for plywood, specifically, press temperature 180?°C, press time 3 min and resin loading 280 g m^?2, type I soy-based plywood could be prepared with a hybrid cross-linker, namely 12%MG + 2% epoxy resin (EPR). The DSC results showed that the reaction between soy-based adhesive and the hybrid cross-linker MG + EPR was very complex.     

Preparation of a New Type of Polyamidoamine and Its Application for Soy Flour-Based Adhesives

The most commonly used curing agents for soy-based adhesives are polyamines, which have the problem of low solid content and/or high viscosity. To overcome this problem, a new type of polyamidoamine (PADA) resin was synthesized and applied to soy flour-based adhesives to improve their water resistance. The PADA solution obtained had a high solid content of 50 wt% and low viscosity of 270 cP. The optimum weight ratio of soy flour/PADA/maleic anhydride to prepare adhesive was 40/7/1.68. The wet strength of plywood prepared at the optimum weight ratio was 0.82 MPa, which meant the plywood could be used as type II plywood according to the Chinese National Standard GB/T 9846.7-2004. The results of water-insoluble solid content measurement and SEM observation demonstrated that cured soy flour–PADA–maleic anhydride adhesive had a 16 % greater water-insoluble solid content than soy flour–NaOH adhesive. The cross-linking network formed by the reactions of PADA and MA would increase the water-insoluble solid contents and improve water resistance of cured soy flour-based adhesives.     

Bond quality of soy-based phenolic adhesives in southern pine plywood

Increased demand for wood adhesives, environmental concerns, and the uncertainty of continuing availability of petrochemicals have led to recent attention on protein-based adhesives. This study was conducted to investigate the glue bond qualities of soy-based phenolic adhesive resins for southern pine plywood. Two types of soy-based resins were formulated and tested. The first was made by cross-linking soy flour with phenol-formaldehyde (pf) resins at neutral pH. The second type was obtained by cross-linking soy flour hydrolyzates with pf resin under alkaline conditions. Plywood bonded with the neutral phenolic soy resins containing 70% soy flour and 30% 1.6 g/cm² pf without the use of extenders and fillers compared favorably with the traditional southern pine pf glue mixes. Plywood bonded with alkaline phenolic soy resins, containing 40 or 50% 0.5 g/cm² PF with the addition of extender (19% corn-cob powder), provided better adhesive glue bond properties than traditional southern pine pf glue mixes. These results suggest that soy-based phenolic adhesive resins have potential for the production of exterior southern pine plywood.     

Combination of lignin polyol–tannin adhesives and polyethylenimine for the preparation of green water‐resistant adhesives

In this study, a green adhesive from renewable lignin and tannin was developed with polyethylenimine (PEI) with a method to improve the water resistance of the lignin/tannin adhesive. Lignin polyols were prepared through the liquefaction of oil‐palm empty fruit bunches. The characteristics of the adhesive samples were compared with those of a commercial phenol–formaldehyde resin. Three plywood specimens bonded with the new adhesive showed a very high tensile strength (63.04 MPa) and were very water resistant. The effect of the solid content of the adhesives on the tensile strength and gel time and various weight ratios of PEI on the tensile strength and water resistance of the plywood specimens were evaluated. Thermal stability tests revealed that the lignin polyol–tannin/PEI adhesives had a high heat resistance (360 °C). © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43437.     

A Soy Flour-Based Adhesive Reinforced by Low Addition of MUF Resin

The desire to prepare a lower-cost soy-based adhesive has led to an interest in using the abundant and inexpensive soy flour (SF) as a substitute for expensive soy protein isolates (SPI) in wood adhesives. However, the weakness of this adhesive is poor water-resistance and bonding strength due to a low protein content, which limits its application in the wood industry. The objective of this research was to provide a simple and useful approach for improving the adhesion performance of SF-based adhesive by introducing a small addition of melamine-urea-formaldehyde (MUF) resin into the cured system. The optimum addition level of MUF resin, as well as the adhesion performance and conformation change of SF-based adhesive, were investigated. The analytical results indicated that the co-condensed methylene bridges were formed through the reaction of methylol groups of MUF resin with soy units during the hot-press process. The addition of MUF resin, not only significantly decrease the viscosity of SF-based adhesive but also increase its water-resistance and wet shear strength value. The SF-based adhesive containing 20% MUF resin, is a relatively low-cost adhesive, has a reasonable viscosity, and moreover can pass the Chinese Industrial Standard requirement (0.7 MPa) for interior plywood panels.     

Synthesis and characterization of phenolic novolacs modified by chestnut and mimosa tannin extracts

The possibility of reacting chestnut and mimosa tannins with the intermediates of the synthesis reaction for phenolic novolacs under acid conditions has been proved using differential scanning calorimetry (DSC). The amount of intermediate compounds and the percentage of free phenol and formaldehyde in the reaction mixture is decisive for the determination of the stage in which the addition of tannin is suitable. Synthesis of novolac resins modified with 14 wt % mimosa tannin extract or with several percentages (until 40 wt %) of chestnut tannin have been performed. The reaction pathways have been investigated by DSC, fourier transformed infrared spectroscopy and gel permeation chromatography. Ester groups of chestnut tannin result in a reaction pathway different from the one for mimosa‐modified resins and nonmodified resins. Preliminary studies of curing reactions of synthesized resins with hexamethylenetetramine indicate that the cure of modified‐resins is even more favorable than the one for nonmodified resins. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4412–4419, 2006     

Using Renewables in Panelboard Resins to Influence Volatile Organic Compound Emissions from Panels

Technical lignin and condensed tannins have been combined with soy flour as model of no-added-formaldehyde adhesive binders for veneer wood products to understand their impacts on volatile organic compounds (VOCs) produced during panel manufacture. VOC emissions captured on manufacturing lauan hardwood plywood at 170?C were dominated by acetaldehyde, hexaldehyde, acetone, and terpenes in both the condensate and gaseous fractions of press emissions. Other aldehydes including formaldehyde, valeraldehyde, and propionaldehyde were produced in relatively lower quantity during panel manufacture. Compared to using soy flour alone, lignin, and tannin reduced the formaldehyde and acetaldehyde contents in press emissions. These reductions in VOCs had a dependency on adhesive resin pH with an alkaline formulation proving to also decrease longer chain aldehydes such as valeraldehyde and hexaldehyde. Chamber testing plywood panels found the composition of VOC emissions initially released from panels to be prominent compounds released in press emissions formed on panel manufacture. Use of soy flour alone as binder produced relatively high acetaldehyde emissions from panels, whereas incorporating lignin and tannin with soy flour as adhesive binders reduced both acetaldehyde and formaldehyde emissions from panelboards post-manufacture.     

High temperature performance of soy-based adhesives

We studied the high temperature performance of soy meal processed to different protein concentrations (flour, concentrate, and isolate), as well as formulated soy-based adhesives, and commercial nonsoy adhesives for comparison. No thermal transitions were seen in phenol-resorcinol-formaldehyde (PRF) or soy-phenol-formaldehyde (SoyPF) or in as-received soy flour adhesive during differential scanning calorimetry scans heating at 10?°C/min between 35 and 235?°C. Heat flow rates decreased in the order soy flour (as received)?>?SoyPF?>?PRF?>?emulsion polymer isocyanate (EPI). In thermogravimetric analysis (TGA) scans from 110 to 300?°C at 2?°C/min, total weight loss decreased in the order soy flour (as-received)>SoyPF?>?PRF?>?casein?>?maple?>?EPI. For bio-based materials, the total weight loss (TGA) decreased in the order soy flour (as-received) > concentrate, casein?>?isolate. Dynamic mechanical analysis from 35 to 235?°C at 5?°C/min of two veneers bonded by cured adhesive showed 30–40% decline in storage modulus for maple compared to 45–55% for the adhesive made from soy flour in water (Soy Flour) and 70–80% for a commercial poly(vinyl acetate) modified for heat resistance. DMA on glass fiber mats showed thermal softening temperatures increasing in the order Soy Flour?<?casein?<?isolate?<?concentrate. We suggest that the low molecular weight carbohydrates plasticize the flour product. When soy-based adhesives were tested in real bondlines in DMA and creep tests in shear, they showed less decrease in storage modulus than the glass fiber-supported specimens. This suggests that interaction with the wood substrate improved the heat resistance property of the adhesive. Average hot shear strengths (ASTM D7247) were 4.6 and 3.1?MPa for SoyPF and Soy Flour compared to 4.7 and 0.8?MPa for PRF and EPI and 4.7 for solid maple. As a whole, these data suggest that despite indications of heat sensitivity when tested neat, soy-based adhesives are likely to pass the heat resistance criterion required for structural adhesives.     

A New Soy Flour-Based Adhesive for Making Interior Type II Plywood

In this study, we developed a formaldehyde-free adhesive from abundant, renewable, and inexpensive soy flour (SF). The main ingredients of this adhesive included SF, polyethylenimine (PEI), and maleic anhydride (MA). The optimum formulation of this adhesive and the optimum hot-press conditions for making plywood were investigated. A three-cycle soak test and a boiling water test (BWT) were employed for evaluating the strength and water-resistance of plywood bonded with this adhesive. Results showed that SF, PEI, MA and sodium hydroxide were all essential components for the adhesive and the SF/PEI/MA weight ratio of 7/1.0/0.32 resulted in the highest water-resistance. When the hot-press temperature was in the range of 140–170 °C, both water-resistance and shear strength of plywood bonded with the adhesive remained statistically the same, except that the dry shear strength of plywood at 170 °C was statistically lower than that at 160 °C. When the hot-press time ranged from 2 to 6 min, the plywood panels at 5 min had the highest boiling water test/wet (BWT/w) shear strength. The plywood panels made at 5 min had a higher dry shear strength than those made at 3 min. Plywood panels bonded with this SF/PEI/MA adhesive exceeded the requirements for interior applications.     

A Formaldehyde-Free Water-Resistant Soy Flour-Based Adhesive for Plywood

The phasing out of the use of urea–formaldehyde adhesive in the fabrication of interior‐used hardwood plywood requires development of environmentally friendly bio‐based wood adhesives. We recently reported that phosphorylation of soy flour (SF) using phosphoryl chloride (POCl₃) greatly improved the moisture resistance of soy flour adhesive. In the present study, we investigated the effects of inorganic oxidizing agents, such as NaClO₂ and Ca(NO₂)₂, to further improve the wet bonding strength of phosphorylated SF (PSF) wood adhesive. We report that addition of 1.8 % (wet weight basis) Ca(NO₂)₂ to phosphorylated SF (PSF) adhesive formulation containing 25 % soy flour solids increased the wet bonding strength to greater than 3 MPa at 140 °C hot‐press temperature. The water resistance testing of the glued three‐ply hardwood plywood panels passed the three‐cycle soak/dry test recommended by the American National Standard for Hardwood and Decorative Plywood/Hardwood Plywood and Veneer Association protocol (ANSI/HPVA HP‐1‐2004). Since the process involves only inorganic chemistry and no petroleum‐based chemicals such as formaldehyde or polyamidoamine–epichlorohydrin are used, the PSF + Ca(NO₂)₂ adhesive is non‐toxic and environmentally safe.     

Phenolic resin adhesives based on chestnut (Castanea sativa) hydrolysable tannins

Chestnut hydrolysable tannins are phenolic materials that have been considered too unreactive to compete in the phenolic resin adhesives market for exterior boards for the building industry. However, an article in 1973 describing 3?years industrial application of chestnut hydrolysable tannins during the first oil crisis indicated that this was not the case. We have extended this old work by using superior phenolic resins formulations and producing phenol–formaldehyde–chestnut tannin adhesives where a substitution of up to 80% of the phenol is possible with remarkably good results. The reactions involved were clarified by ¹³C NMR and MALDI-TOF mass spectrometry.     

CHT and TTT curing diagrams of polyflavonoid tannin resins

TTT and CHT curing diagrams for tannin-based adhesives were built by thermomechanical analysis (TMA) by following the in situ hardening directly in a wood joint, and the curve trends observed were similar to those previously observed for synthetic polycondensation resins on lignocellulosic substrates. Of the parameters that most influence the relative position of vitrification and gel curves on the diagrams (i.e., where the influence has been quantified), chief among them is the reactivity of the tannin with formaldehyde and any factor influencing it: thus, the inherent higher reactivity of the A-ring of the tannin (such as in procyanidins versus prorobinetinidins) and the pH of the tannin solution. The percentage formaldehyde hardener has some influence in CHT diagrams, especially for the slower-reacting tannins, but practically no influence in TTT diagrams within the 4–10% formaldehyde range used. As in the case of synthetic polycondensation adhesive resins, regression equations relating the internal bond strength of a wood particleboard, prepared under controlled conditions, with the inverse of the minimum deflection, obtained by constant heating rate TMA of a wood joint during resin cure, have been obtained for the two types of tannins of lower reactivity (profisetinidins/prorobinetinidins) but not for the faster-reacting procyanidin and prodelphinidin tannins. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 3220–3230, 2001     

Shear Refinement of Formaldehyde-Free Corn Starch and Mimosa Tannin (Acacia mearnsii) Wood Adhesives

The aim of this work was to reduce the viscosity of formaldehyde-free corn starch–mimosa tannin wood adhesives, without adversely affecting the mechanical properties of the product. The reduction of viscosity was achieved using shear refinement. The study focused on the physical phenomena before cross-linking of the wood adhesive. The physical (rheological characterization) and mechanical (bond strength) properties of formaldehyde-free corn starch and mimosa tannin wood adhesives were measured. The results showed that the shear refinement (290 rpm and 5 min, optimal conditions) reduced the viscosity of the corn starch–mimosa tannin wood adhesives (from 100 000 to 458 Pa s) with the advantage of being stable over time. Mechanical tests showed that the shear refinement did not influence the mechanical properties of corn starch–mimosa tannin wood adhesives.     

Chromatographic Analysis of the Reaction of Soy Flour with Formaldehyde and Phenol for Wood Adhesives

The desire to make more biobased and lower-cost bonded wood products has led to an interest in replacing some phenol and formaldehyde in wood adhesives with soybean flour. Improved knowledge of the soy protein properties is needed to relate resin chemistry to resin performance before and after wood bonding. To expose the soy protein’s functional groups, it needs to be disrupted, with minimal hydrolysis, to maximize its incorporation into the final polymerized adhesive lattice. The best conditions for alkali soy protein disruption were to maintain the temperature below 100 °C and react the soy flour with sodium hydroxide at pH 9–12 for about 1 hour. A gel permeation chromatography procedure was optimized to determine conditions for selectively breaking down the high molecular weight soy protein fragments that contribute to high adhesive viscosity. This method and extraction data were used to evaluate the reaction of the disrupted soy flour protein with formaldehyde and phenol to provide a stable adhesive. The results were used to develop more economical adhesives that are ideally suited for the face section of oriented strandboard.     

Mussel-inspired bio-based water-resistant soy adhesives with low-cost dopamine analogue-modified silkworm silk Fiber

Mussel-inspired dopamine chemistry is popular among engineers for surface modification on various substrates due to its high efficiency, handy operation process, and strong reactivity. However, the high cost of dopamine does not allow for mass production. In the present study, low-cost dopamine analogues (alkali lignin and tannic acid) were used to fabricate high-reactivity silkworm silk fiber (SF) via a simple dip-coating approach, and were then applied to a soy-based adhesive to enhance its performance. The SF tightly combines with soy protein mainly via a Schiff base reaction between polydopamine or dopamine analogue and the amine or thiol groups of soy protein; this forms a multiple crosslinked system and “reinforced concrete”-like structure, as confirmed by Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetry, and scanning electron microscopy analyses. As expected, the toughness of the soy-based adhesive obviously improved and the highest wet shear strength of the adhesive samples attained 1.50 MPa, which is far greater than relevant interior use requirements. Though dopamine-coated SF could significantly enhance the wet shear strength of the soy-based adhesive by 387.1% compared to the pristine SM adhesive, lignin-coated and tannic acid-coated SFs are more suitable for practical application due to the lower cost of raw materials. The results of this study may represent an effective and low-cost approach to mussel-inspired surface modification chemistry for the mass production of high-performance soy-based adhesives. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48785.     

Effects of Pinus pinaster bark extracts content on the cure properties of tannin-modified adhesives and on bonding of exterior grade MDF

Phenol-urea-formaldehyde-tannin (PUFT) adhesives with different degrees of phenol substitution by Pinus pinaster bark tannins were thermally characterized by dynamic mechanical thermal analysis (DMTA) and by differential scanning calorimetry (DSC). They were employed to prepare exterior grade Medium Density Fiberboards (MDF) according to European Standards. DMTA and DSC experiments showed, first, that the tannin-modified adhesives hardened in the low temperature range (30-110°C) and, second, that increasing the tannin content of the adhesives reduced the curing temperature, obtaining at least the same mechanical strength (stiffness) and higher curing enthalpies (ΔH) than the commercial phenolic resin. Although only the MDF boards made using the lowest viscosity tannin-modified adhesive (PUFT-10), with a 44% phenol replacement by tannin, met the outdoor requirements, all the other tannin-modified adhesives boards met the interior grade specifications. Among the board properties evaluated, the low value of thickness swelling after 24-h water immersion of MDF boards prepared using the PUFT-10 (6.6%) is particularly noticeable, which means an improvement of almost 50% compared to that of the commercial PF (12.6%).     

Low Formaldehyde Emitting Biobased Wood Adhesives Manufactured from Mixtures of Tannin and Glyoxylated Lignin

Abstract

Kraft (LN-T-CO₂-2) and wheat straw (CIMV) glyoxalated lignin mixed with mimosa tannin and hexamine as a hardener were used as wood adhesive resins in particleboard fabrication. The adhesive systems proportion used were 40/60 and 50/50 w/w for lignin and tannin, respectively. The gel time test was determined by knowing the polymerization time between the different mixes under the controlled conditions. The results showed a slower polymerization with the kraft lignin/mimosa tannin blending than with the wheat straw lignin/mimosa tannin one. Thermomechanical analyses (TMA) tests were carried out as an indication of the final strength of the adhesive systems revealed by the elasticity modulus (MOE). The MOE results have demonstrated the best mechanical resistance values in 40/60 lignin/mimosa tannin proportion with respectively 3.422 and 3.347 (MPa), for CIMV and LN-T-CO₂-2, and 2.122 (MPa) for 50/50 proportion. Particleboards were prepared and the internal bond (IB) tests were carried out according to the European Standard EN 312. The IB tests confirmed the TMA results. The higher mechanical results of the IB were .43 and 0.53 (MPa), for CIMV and LN-T-CO₂-2 lignin in a 40/60 lignin/mimosa tannin proportion. They were classified as interior panel P2 in according with the standard request EN-312. Free-formaldehyde was determined through the flask method EN 717-3. Particleboards prepared with these natural adhesive resins registered emissions at least 87 and 75% lower than the commercial UF and MUF dhesive resins. The panels were classified as E0.     

A new environmentally friendly bioadhesive mainly modified by propanetriol

In this paper, a series of new environmentally friendly bioadhesives with improved bonding strength were quickly synthesized via urea, sodium dodecyl sulfate (SDS) and propanetriol are mixed with soy isolate protein. The results showed that the bonding strength of the modified adhesives was changed with the increasing content of propanetriol. The maximum dry shear strength of the plywood bonded with the resultant adhesive was increased to 2.45 MPa when the propanetriol content was 20 ml. While the maximum wet shear strength of the plywood bonded with the resultant adhesive arrived 1.32 MPa, which is acceptable for industrial application in plywood fabrication according to the national standards of the People’s Republic of China (≥0.7 MPa). In addition, the orthogonal experiment suggested that the obtained material with pH of 9 for 5 h mixing at the hot pressing temperature of 120 °C exhibited the best comprehensive performance. Also, the FTIR, SEM and DSC measurements showed that the adhesives had a compact structure with stable thermal property.     

Soybean meal-based adhesive enhanced by MUF resin

Soybean meal flour, polyethylene glycol (PEG), sodium hydroxide (NaOH), and a melamine-urea-formaldehyde (MUF) resin were used to formulate soybean meal/MUF resin adhesive. Effects of the adhesive components on the water resistance and formaldehyde emission were measured on three-ply plywood. The viscosity and solid content of the different adhesive formulations were measured. The functional groups of the cured adhesives were evaluated. The results showed that the wet shear strength of plywood bonded by soybean meal/NaOH adhesive increased by 33% to 0.61 MPa after adding NaOH into the adhesive formulation. Addition of PEG reduced the viscosity of the soybean meal/NaOH/PEG adhesive by 91% to 34,489 cP. By using the MUF resin, the solid content of the soybean meal/MUF resin adhesive was improved to 39.2%, the viscosity of the adhesive was further reduced by 37% to 21,727 cP, and the wet shear strength of plywood bonded by the adhesive was increased to 0.95 MPa, which met the interior plywood requirements (≥0.7 MPa). The formaldehyde emission of plywood bonded by the soybean meal/MUF resin adhesive was obtained at 0.28 mg/L, which met the strictest requirement of the China National Standard (≤0.5 mg/L). FTIR showed using the MUF resin formed more  CH₂ group in the cured adhesive. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Soy-based adhesives for wood-bonding – a review

Over recent years, the interest in bio-adhesives, including soy-based adhesives, has increased rapidly. Among natural renewable resources suitable for industrial use, soy is a reasonable choice due to its high production volume and the small use of soy meal-based products for human food consumption. Soy flour can be an ideal raw material for the manufacturing of wood adhesives due to its low cost, high protein content and easy processing. There are also more concentrated forms of soy proteins, i.e. concentrates and isolates, which are also suitable raw materials for adhesive production except that their prices are higher. Extensive research has been carried out on improving the cohesive properties, especially water resistance, of soy-based adhesives. However, there is insufficient experimental data available for understanding the influences of modification methods on the structure of soy proteins and therefore for understanding the influences of structural changes on the adhesion. In this paper, some experimental techniques are proposed to be used for analysing soy-based adhesives to enable better understanding of those factors and improve future development. This review of soy-based adhesives is made with the focus on soy proteins’ chemical composition, soy protein product types (raw materials for adhesive production), modification methods for improving the adhesive properties of soy-based adhesives, and commercial soy-based adhesives.